Bitumen is a heavy type of crude oil that is often found in naturally occurring geological materials such as tar sands, black shales, coal formations, and weathered hydrocarbon formations contained in sandstones and carbonates. Bitumen may be described as flammable brown or black mixtures or tarlike hydrocarbons derived naturally or by distillation from petroleum. Bitumen can be in the form of a viscous oil to a brittle solid, including asphalt, tars, and natural mineral waxes. Substances containing bitumen may be referred to as bituminous, e.g., bituminous coal, bituminous tar, or bituminous pitch. At room temperature, the flowability of bitumen is much like cold molasses. Bitumen may be processed to yield oil and other commercially useful products, primarily by cracking the bitumen into lighter hydrocarbon material.
As noted above, tar sands represent one of the well known sources of bitumen. Tar sands typically include bitumen, water, and mineral solids. The mineral solids can include inorganic solids such as coal, sand, and clay. Tar sand deposits can be found in many parts of the world, including North America. One of the largest North American tar sands deposits is in the Athabasca region of Alberta, Canada. In the Athabasca region, the tar sands formation can be found at the surface, although it may be buried two thousand feet below the surface overburden or more.
Tar sands deposits can be measured in barrels equivalent of oil. It is estimated that the Athabasca tar sands deposit contains the equivalent of about 1.7 to 2.3 trillion barrels of oil. Global tar sands deposits have been estimated to contain up to 4 trillion barrels of oil. By way of comparison, the proven worldwide oil reserves are estimated to be about 1.3 trillion barrels.
The bitumen content of some tar sands may vary from approximately 3 wt % to 21 wt %, with a typical content of approximately 12 wt %. As such, an initial step in deriving oil and other commercially useful products from bitumen typically may require extracting the bitumen from the naturally occurring geological material. In the case of tar sands, this may include separating the bitumen from the mineral solids and other components of tar sands.
One conventional process for separating bitumen from mineral solids and other components of tar sands includes mixing the tar sands with hot water and, optionally, a process aid such as caustic soda (see, e.g., U.S. Pat. No. 1,791,797). Agitation of this mixture releases bitumen from the tar sands and allows air bubbles to attach to the released bitumen droplets. These air bubbles float to the top of the mixture and form a bitumen-enriched froth. The froth may include around 60% bitumen, 30% water, and 10% inorganic minerals. The bitumen-enriched froth is separated from the mixture, sometimes with the aid of a solvent, and further processed to isolate the bitumen product. For example, the froth may be treated with an aliphatic (pentane-type) or an aromatic (naphtha-type) solvent to produce a clean bitumen product that may serve as a refinery upgrader feed stock. The bulk of the mineral solids can also be removed to form a tailings stream. The tailings stream may also include water, solvent, precipitated asphaltenes (in the case where the asphaltene is not soluble in the solvent used to separate the bitumen-enriched froth from the mixture), and some residual bitumen.
Tailings produced by the hot water process and/or the froth treatment process pose several problems. Firstly, as noted above, the tailings produced by conventional methods may include precipitated asphaltenes and/or residual bitumen. The bitumen and asphaltenes in a tailings stream represent unrecovered hydrocarbon that will not be processed into valuable commercial products. Accordingly, the conventional methods result in a lower yield of hydrocarbon material, and consequently, diminished profit.
Additionally, the presence of bitumen and asphaltene in the tailings may complicate the disposal of the tailings because theses hydrocarbons can present environmental risks. This may also be true for residual solvent included in the tailings that may be environmentally unfriendly.
The amount of tailings produced by conventional methods may also present significant problems. In some circumstances, the total volume of the tailings produced by the conventional methods may be more than the volume of mined tar sands, which means that not all of the tailings can be returned to the mined area.
The physical characteristics of the tailings may also present significant problems. The conventional methods utilize water and caustic as part of the process. This may result in the activation and swelling of certain clay components of a tailings stream. As such, the tailings have a sludge-like consistency that can last indefinitely. The sludge-like consistency means that the tailings are not stackable, thereby severely limiting the manner in which to dispose of the tailings. Often the only disposal option is to deposit the tailings in a tailings pond located outside of the mine area. These ponds are costly to build and maintain and can be damaging to the local environment, including the local water supply. Furthermore, ponds can be damaging to the local wildlife population, such as birds, which can be caught in the oil and solvent laden tailings produced by hot-water extraction processes.
One known method for separating bitumen from tar sands that also produces a tailings stream is described in U.S. Pat. No. 4,347,118 (the '118 patent). The '118 patent discloses a method in which pentane is used to extract bitumen from tar sands. The pentane solvent does not dissolve the asphaltene fraction of the bitumen that is not pentane soluble. Thus, this fraction of the bitumen remains as a solid and is discharged with the tailings. The precipitated C5 asphaltene possesses a complex, card-house type structure that tends to include bitumen and/or solvent, resulting in additional losses of hydrocarbons with the tailings. For Athabasca-type bitumen, the asphaltene precipitate alone can result in a significant loss of the total initial hydrocarbon content of the tar sands, and original bitumen can also be lost to tailings due to inclusions in the asphaltene precipitate.
U.S. Pat. No. 5,143,598 (the '598 patent) discloses a method that includes adding heptane to tar sands to form a bitumen-rich heptane phase and then displacing the bitumen-rich heptane phase with water. The use of heptane can result in the precipitation of the heptane insoluble C7 asphaltene fraction present in the bitumen phase. The heptane insoluble asphaltene fraction is discharged with the tailings. In addition, using water not only results in undesirable swelling of clays in the tar sands, but also can form undesirable emulsions with the lighter hydrocarbon fraction of bitumen that are quite difficult to break.